The present invention relates to a pressure sensing device, and more particularly, to a pressure sensing device incorporating an optically powered resonant integrated microstructure (O-RIM).
In a typical O-RIMS (optically powered resonant integrated microstructure) device, a microbeam having a resonant frequency is fastened to the shell by two supports and is vacuum encapsulated by a polysilicon shell. The microbeam and the shell are supported by a silicon substrate, all of which together form a micromachined integrated silicon device. A typical O-RIMS device is further provided with an optical fiber which is positioned in proximity to the resonant microbeam.
The shell, the microbeam and the substrate create a set of Fabry-Perot cavities, such that light reflected from these surfaces interfere with one another as they re-enter the optical fiber, creating an optical signal whose intensity changes as the microbeam moves up and down. Thus, the beat frequency of the reflected light indicates the frequency of vibration of the microbeam.
Light generated by a light emitting diode (LED), a laser or other light source arrives at the O-RIMS device via the optical fiber, passes through the shell, partially through the microbeam, and on to a photodiode situated beneath the resonant microbeam. The shell is partially reflective and partially transparent to the arriving light. A portion of the light passing through the shell is reflected from the microbeam, through the shell, and back into the optical fiber.
The microbeam is excited to resonance by the arrival of the light though the optical fiber striking the photodiode causing charge to build up there, creating an electrostatic attraction to the microbeam. The electrostatic attraction causes the microbeam to flex, and as the microbeam approaches its maximum flexure, its potential energy builds to a point where its restoring force overcomes the electrostatic attraction. The microbeam then springs toward a neutral or resting position, where the electrostatic attraction builds again, flexing the microbeam again, and exciting resonance in the microbeam.
The basic premise of an O-RIMS pressure sensor is that, by monitoring the resonant frequency of the resonant beam, the pressure in the medium surrounding the shell of the device can be determined, because the resonant frequency of the microbeam changes when the beam""s supports are moved further apart or brought closer together. Therefore, when the shell deforms under the applied pressure, the supports move further apart or are brought closer together, thereby causing the resonant frequency of the microbeam to change.
In earlier O-RIMS pressure sensing devices, a single wavelength of light was used both to drive the microbeam into resonance and to detect the motion of the microbeam. In an alternative version of the earlier devices, one wavelength of light was used to drive the device, and another was used to detect vibration of the microbeam. In either case, detection of microbeam vibration was achieved by illuminating a relatively broad area around the microbeam, and then detecting changes in the intensity of light caused by the motion of the microbeam. Therefore, if the microbeam area is only a small fraction of the total illuminated area, it is very difficult to find the signal amidst all of the background light/noise. As a practical matter, the optical fiber should be very close (e.g., a few tens of microns) to the microbeam to insure that an adequate signal to noise ratio is achieved.
The pressure sensor of the present invention improves upon the pressure sensing capabilities of earlier such devices by placing a fluorescent material (e.g. erbium) under a portion of the microbeam, such that a Fabry-Perot cavity comprising the erbium coated substrate, the microbeam, and the shell is formed.
Changing dimensions of the Fabry-Perot cavity causes light that escapes from the device to be modulated as the resonant frequency of the microbeam changes in response to pressure on the shell. Since virtually all of the fluorescent light has to pass through the Fabry-Perot cavity to get to a light transporter, such as an optical fiber or an optical waveguide, the signal at the light transporter is strongly modulated. Thus, it is relatively easy to detect the change in vibratory motion of the microbeam.
Accordingly, the present invention offers a very high signal-to-noise ratio when a fluorescent material such as erbium is used. This very high signal-to-noise ratio is due to the fact that erbium fluoresces at 1.55 microns when illuminated at 900 nm, unlike few naturally occurring materials. Moreover, black body radiation in room temperature objects is very low at 1.55 microns.
In addition, the present invention is practically immune to background noise in many applications, because the signal from the modulated erbium at a wavelength of 1.55 microns is low. However, the background noise is low too, thereby further accounting for the high signal-to-noise ratio.
The present invention requires no external electric power because the optical power required to drive the microbeam into resonance is very low (estimated to be in the nanowatt range). Thus, it is practical to power this device only with light. Moreover, since no electrical power is required, device packaging is greatly simplified, and the operative component of the device can comprise the O-RIMS structure on an appropriately designed die bonded directly to the tip of an optical fiber. Hence, the complete sensor can have a diameter no bigger than the tip of the optical fiber.
In addition, the sensors can be mass produced cheaply using microelectronic machining system (MEMS) technology.
In accordance with one aspect of the present invention, a pressure sensing device comprises a substrate having a fluorescent region, a shell having an outer surface and an inner surface, a beam affixed to the inner surface of the shell by two posts, a first light transporter having a distal end and an end proximate the outer surface of the shell in an area adjacent the beam and the fluorescent region, and a second light transporter having a distal end and an end proximate the outer surface of the shell in an area adjacent the beam and the fluorescent region.
In accordance with another aspect of the present invention, a method for sensing pressure using a vacuum cavity device having at least one fluorescent region and a pressure sensitive resonant beam comprises directing a first light wave toward the pressure sensitive resonant beam and the fluorescent region, exciting the pressure sensitive resonant beam to a resonant frequency in response to the first light wave, and transmitting away from the pressure sensitive resonant beam a second light wave generated by the fluorescent region in response to the first light wave, the first and second light waves having different wavelengths, the second light wave having a property corresponding to the resonant frequency of the pressure sensitive resonant beam.
In accordance with a further aspect of the present invention, an optically powered integrated microstructure remote pressure sensor comprises a substrate, a microbeam, a, photodiode, and first and second light transporters. The substrate supports a polysilicon shell having an outer surface and an inner surface, the inner surface defines an evacuated cavity enclosing an area of the substrate, and the substrate is provided with a fluorescent region. The microbeam is affixed to the inner surface of the shell within the evacuated cavity by two spaced apart posts, and the microbeam is disposed in the vicinity of the substrate. The photodiode is integrated into the substrate at a surface location beneath the microbeam. The first light transporter has a distal end and a proximate end, and the proximate end of the first optical fiber is disposed adjacent the outer surface of the shell to direct light from the first optical fiber to the photodiode and to the fluorescent region. The second light transporter has a distal end and a proximate end, the proximate end of the second light transporter is disposed at the outer surface of the shell so as to transmit light emitted by the fluorescent region, and the transmitted light indicates a frequency of vibration of the beam.
In accordance with yet a further aspect of the present invention, a pressure sensing device comprises a substrate having a photodiode and a fluorescent region, a shell having an outer surface and an inner surface, a beam affixed to the inner surface of the shell by two posts, and a light transporter having a distal end and an end proximate the outer surface of the shell in an area adjacent the beam and the fluorescent region.
In accordance with still a further aspect of the present invention, a pressure sensing device comprises a substrate having a photodiode and a fluorescent region, a shell having an outer surface and an inner surface, and a beam affixed to the inner surface of the shell by two posts.
In accordance with an additional aspect of the present invention, an optically powered integrated microstructure remote pressure sensor comprises a substrate, a microbeam, a photodiode, and a light transporter. The substrate supports a polysilicon shell having an outer surface and an inner surface, the inner surface defines an evacuated cavity enclosing an area of the substrate, and the substrate is provided with a fluorescent region. The microbeam is affixed to the inner surface of the shell within the evacuated cavity by two spaced apart posts, and the microbeam is disposed in the vicinity of the substrate. The photodiode is integrated into the substrate at a surface location beneath the microbeam. The light transporter has a distal end and a proximate end, the proximate end is disposed adjacent the outer surface of the shell to direct light to the photodiode and to the fluorescent region and to receive a light wave emitted by the fluorescent region, and the transmitted light indicates a frequency of vibration of the beam.